202 NATUEAL HISTOEY BULLETIN 



The numbers 0, 2, 4, 6, 8, 10, 12, and 14 on the left hand side 

 of each figure represent wind- velocity in miles per hour ; the cor- 

 responding decimals represent evaporation from the Piche evap- 

 orimeter measured in tenths of a cubic inch ; and the degrees Fah- 

 renheit are indicated on the right hand side of each figure. 



A striking relation between evaporation and wind velocity is 

 shown by these figures. While the temperature is in most cases 

 comparatively regular the curves representing wind velocity and 

 amount of evaporation are more or less irregular, but correspond 

 closely to each other in their irregularities. This is especially no- 

 ticeable in the afternoon when both wind and sun are most effec- 

 tive. The close relation between wind velocity and evaporation 

 is more pronounced as the day advances, and it also became more 

 striking as the dry season advanced and the relative humidity of 

 the air diminished. 



An inspection of the relative humidity table for station 1 

 shows the following variation : 



Rains were frequent during the first half of August, but very 

 little rain fell between the foregoing dates. 



The curve for September 5th also shows that both wind and 

 temperature must operate together to cause the maximum evap- 

 oration, for on this day the wind was quite brisk early in the 

 morning, yet evaporation was slow until the temperature had in- 

 creased sufficiently to induce great evaporation, the function of 

 the wind evidently being the removal of the vapors as they are 

 formed, — thus making room for more. In the same connection 

 it is interesting to compare the amount of evaporation at night 

 and in the daytime. It will be observed that at night evaporation 

 at the several stations was more nearly the same than in the day- 

 time, for the exposure of the several stations was then more nearly 

 equal, the sun having disappeared and the wind having died 

 down. 



The foregoing facts suggest an explanation of the presence of 



